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Characteristics of the bioreactor landfill system using an anaerobic–aerobic process for nitrogen removal
Bioresource Technology 98 (2007) 2526–2532
Characteristics of the bioreactor landfill system using an anaerobic–aerobic process for nitrogen removal Ruo He a
a,b,*
, Xin-wen Liu
b,c
, Zhi-jian Zhang b, Dong-sheng Shen
b
Ministry of Education Key Lab of Environment Remediation and Ecological Health, Zhejiang University, Hangzhou 310029, China b College of Environment and Resource, Zhejiang University, Hangzhou 310029, China c Department of Chemical engineering, Ningbo University of Technology, 20 Cuibai Road, NingBo 315016, PR China Received 18 April 2006; received in revised form 5 September 2006; accepted 10 September 2006 Available online 27 October 2006
Abstract A sequential upflow anaerobic sludge blanket (UASB) and air-lift loop sludge blanket (ALSB) treatment was introduced into leachate recirculation to remove organic matter and ammonia from leachate in a lab-scale bioreactor landfill. The results showed that the sequential anaerobic–aerobic process might remove above 90% of COD and near to 100% of NHþ 4 -N from leachate under the optimum organic loading rate (OLR). The total COD removal efficiency was over 98% as the OLR increased to 6.8–7.7 g/l d, but the effluent COD concentration increased to 2.9–4.8 g/l in the UASB reactor, which inhibited the activity of nitrifying bacteria in the subsequent ALSB reactor. The NO 3 -N concentration in recycled leachate reached 270 mg/l after treatment by the sequential anaerobic–aerobic process, but the landfill reactor could efficiently denitrify the nitrate. After 56 days operation, the leachate TN and NHþ 4 -N concentrations decreased to less than 200 mg/l in the bioreactor landfill system. The COD concentration was about 200 mg/l with less than 8 mg/l BOD in recycled leachate at the late stage. In addition, it was found that nitrate in recycled leachate had a negative effect on waste decomposition. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Anaerobic–aerobic process; Municipal solid waste; Bioreactor landfill; Biological nitrogen removal
1. Introduction Leachate recirculation is a new innovative practice in municipal solid waste (MSW) management because it reduces leachate organic strength, accelerates landfill stabilization, reduces landfill active life and increases landfill gas production (Reinhart and Townsend, 1998). However, recent findings have indicated that this practice negatively impacts the landfill environment. The increased level of biodegradation associated with leachate recirculation could result in the imbalance of the growth rates between fastgrowing acidogenic bacteria and slow-growing methano-
* Corresponding author. Address: Ministry of Education Key Lab of Environment Remediation and Ecological Health, Zhejiang University, Hangzhou 310029, China. Tel./fax: +86 571 86971156. E-mail address: [email protected] (R. He).
0960-8524/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2006.09.013
gens in the first phases of waste decomposition. As a result, methanogenesis may be delayed, prevented, or inhibited (O’Keefe and Chynoweth, 2000). In addition, with the waste decomposition and leachate recirculation, ammonia tends to accumulate and reach high concentrations in the anaerobic bioreactor landfill, which would inhibit anaerobic biodegradation of organic waste (Tada et al., 2005; Calli et al., 2006). The C/N ratio is lower in the stabilized leachate under the most conditions, and this has caused serious challenges to treatment systems (Trabelsi et al., 2000). Leachate ammonia is ordinarily removed by the way of ex situ treatment for bioreactor landfills. The methods for ammonia removal from stabilized leachate include ion exchange, air-stripping, chemical precipitation, reverse osmosis and biological nitrogen removal. Among these, biological treatment is the most common method for ammonia removal in leachate, due to the lower cost (Jokela
R. He et al. / Bioresource Technology 98 (2007) 2526–2532
et al., 2002; Uygur and Kargi, 2004). Conventional systems of biological nitrogen removal involve both nitrification and denitrification. For the high ammonia/low carbon wastewaster, organic matter limitation for denitrification appears, which is usually overcome by addition of external carbon sources such as methanol (Shiskowski and Mavinic, 1998). On the other hand, the process of biological nitrogen removal for leachate usually needs air-stripping pretreatment to adjust C/N ratio (Uygur and Kargi, 2004). This not only increases operational costs, but also enhances the difficulty of management. Recently, laboratory studies have showed the efficacy of in situ nitrogen removal in solid waste environment. Zhao et al. (2002) reported that 99.5% of the leachate ammonia was removed in a biofilter consisting of old waste (8–10 years old) with both anaerobic and aerobic sections. Onay and Pohland (1998) developed a three-component simulated landfill system, including anoxic, anaerobic and aerobic zones, to demonstrate the feasibility of in situ nitrification and denitrification at controlled landfills operated with leachate recirculation. The results demonstrated that both separate and combined reactor operations with international leachate recycling around each reactor provided 95% nitrogen conversion. In contrast, combined reactor operation, without internal recycling had conversion efficiency per cycle ranging from 30% to 52% for nitrification and from 16% to 25% for denitrification. Experiments by Berge et al. (2006) indicated that nitrification and denitrification may occur simultaneously in one aerobic landfill cell (even under low biodegradable C/N conditions). A landfill as a bioreactor for the conversion of nitrate to a harmless byproduct, nitrogen gas, is technically viable, and the denitrification reaction did not adversely affect the leachate pH (Price et al., 2003). The effectiveness of a combined ex situ nitrification and in situ denitrification process for managing nitrogen in leachate has been investigated (He et al., 2006). However, to date, landfill bioreactors for nitrogen removal from leachate with the variation of the quality and volume during landfill stabilization has been poorly understood. In the present study, the chemical oxygen demand (COD) and NHþ 4 -N removals were investigated in the anaerobic–aerobic process during landfill stabilization. In addition, the effects of an anaerobic–aerobic treatment in recycled leachate on nitrogen removal from leachate and waste stabilization were evaluated, compared to the bioreactor landfill system only with an anaerobic reactor using leachate recirculation. 2. Methods 2.1. MSW composition MSW was mixed by 10 different components shredded into 2–4 cm pieces in the experiment. The physical composition of the synthetic MSW mixture, according to the investigation made in the city of Ningbo, was as follows
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(by weight): vegetables, 45.0%; fish, 2.5%; meat, 1.0%; fruit, 9.0%; paper, 7.4%; plastics and leather rubber, 11.9%; cellulose textile, 3.6%; brick sand and soil, 8.5%; metals and glasses, 7.6%; woods, 3.5%. The moisture content of the mixture was about 54% (w/w). 2.2. Experimental set-up Three types of reactors used in this study are illustrated in Fig. 1. The simulated landfill reactor consisted of a 42-l cylinder made of PVC (28.7-cm i.d., 65-cm height). A polyethylene male adapter (about 0.8 cm) was installed at the bottom of each landfill reactor as a leachate drainage port. Two such adapters were installed in the lid of each landfill reactor for leachate recirculation and gas collection. Adapters were held in place with wax to provide a gas-tight system. The anaerobic reactor was operated in an upflow anaerobic sludge blanket (UASB) made of PVC (10-cm i.d., 80-cm height) with a working volume of 5.50 l. The aeration reactor was operated in an air-lift loop sludge blanket (ALSB), of which the working volume was 5.24 l. Several collection tanks with individual volume of about 3 l were attached to landfill, UASB and ALSB reactors for collections of leachate and effluent. 2.3. Experimental design and operation Two landfill reactors were used in the experiment. One was connected sequentially with a UASB reactor and an ALSB reactor by leachate recirculation in the experimental bioreactor landfill system. Fig. 1 shows its operational configuration. The other, only combined with a UASB reactor, was used as the control. The UASB reactors were seeded with raw anaerobic sludge procured from the Hangzhou citric acid factory and Hangzhou Shibao sewage treatment plant and 4.5 l of sludge was added to each of UASB reactors. The sludge was incubated with the synthetic water having a COD of 3000–5000 mg/l for 10 days to activate the sludge activity. Then it was acclimated by leachate from Hangzhou Tianzhiling landfill. The acclimated sludge in the UASB reactors of the bioreactor landfill system and the control contained total solids (TS) content of 107 g/l and 96.5 g/l and volatile solids (VS) content of 37.0 g/l and 34.5 g/l, respectively. The ALSB reactor was seeded with raw aerobic sludge procured from Hangzhou Shibao sewage treatment plant and 6 l of sludge, containing TS content of 14.0 g/l and VS content of 8.0 g/l, was added to the reactor. An aeration rate of 0.002 l/min m3 was introduced at the bottom of ALSB reactor. The dissolved oxygen (DO) was kept at 5 mg/l or so in the upflow zone, and about 3.8 mg/l in the downflow zone during the entire experiment. The sludge was acclimated by leachate with COD concentration of 200 mg/l and NHþ 4 -N concentration adjusted from 178 to 300 mg/l by NH4Cl. After ammonia removal efficiency reached 75% under the condition of the influent ammonia
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Wet gas meter
Pump
Sand
Collection tank Aeration
MSW Thermometer
Landfill reactor
UASB reactor
ALSB reactor
Fig. 1. Schematic diagram of the bioreactor landfill system in the experiment.
concentration and loading rate of 300 mg/l and 0.14 g/l d, the start-up of ALSB reactor was completed. Prior to filling, a 5 cm thickness of gravel was placed at the bottom of each landfill reactor to retain refuse and stop small particles from leaching out. Then about 23.4 kg synthetic MSW mixture, which was added with deionized water to 75% moisture content, was filled into each landfill reactor and a specific height of 50 cm was attained. Finally, the waste mixture was covered with a 5 cm depth of sand. Leachate was first treated in the UASB reactor, and the effluent subsequently was fed to the ALSB reactor. The effluent discharged from the ALSB reactor was recirculated into the landfill reactor for denitrification. Leachate was continuously circulated among the landfill, UASB and ALSB reactors by using pumps with adjusted flow rates changing with leachate volume during waste decomposition, except for the first two days when no recycled lechate was fed to the landfill reactor. The simulated landfill reactors were operated at room temperature and the UASB and ALSB reactors were carried out in a temperature-controlled room at 30 ± 1 °C. 2.4. Analytical methods Samples were collected weekly from the effluent port of each reactor. COD, total nitrogen (TN), NHþ 4 -N, NO3 -N and pH were determined by conventional methods (EPA of China, 1989). Volatile fatty acids (VFA) were analyzed by acidified ethylene glycol colorimetric method (CBICAS, 1984).
Fig. 2. OLRs, COD concentrations and COD removal efficiencies in the sequential anaerobic–aerobic process.
3. Results and discussion 3.1. COD removal in the sequential anaerobic–aerobic process Fig. 2 shows the temporal variations of organic loading rates (OLRs), COD concentrations and COD removal efficiencies in the sequential anaerobic–aerobic process. With the degradation of landfilled refuse, the influent COD concentration in the UASB reactor increased at the early stage, and reached the maximum value of 52.7 g/l on day 21, then
decreased to 560 mg/l on day 105. The curve of OLRs changing over time in the UASB reactor was similar to that of COD concentration. However, the highest OLR occurred on day 28, which was later in a week compared with the highest influent COD concentration. This could be attributed to more leachate produced on day 28 than that on day 21, due to waste decomposition. When the OLR was increased to 6.8–7.7 g/l d between days 28 and 35, the effluent COD concentration reached 2.9–4.8 g/l, although the COD removal efficiency was still above 88%
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in the UASB reactor. Previous studies have also shown that COD concentration in effluent discharged from anaerobic reactor was increased while OLRs were increased step by step (Im et al., 2001). The COD removal efficiency was increased to 98% as the OLR increased from 0.04 to 0.95 g/l d in the ALSB reactor. The total COD removal efficiency in the whole system was not affected by the increase of influent OLR, and remained above 90% during the first 63 days. When the influent COD concentration decreased to below 1000 mg/l in the UASB reactor, the COD removal efficiency dropped to about 14–40%. This was consistent with previous report in anaerobic treatment of leachate generated from old and young landfills (Me´ndez et al., 1989). Since the residual COD in the effluent discharged from the UASB reactor was continuously metabolized in the ALSB reactor, the total COD removal efficiency was maintained above 65% in the anaerobic–aerobic process. The effluent COD concentration remained 200 mg/l or so in the ALSB reactor at the late stage, due to a low biodegradability of leachate organics with less than 8 mg/l biochemical oxygen demand (BOD) (data not shown). This was similar with the effluent inert COD concentration in the sequential two-stage UASB/ completely stirred tank reactor (CSTR) system for the treatment of landfill leachate (Ag˘dag˘ and Sponza, 2005). 3.2. NHþ 4 -N removal in the sequential anaerobic–aerobic process NHþ 4 -N
NHþ 4 -N
The variations of loading rates, concentrations and NHþ -N removal efficiencies in the sequen4 tial anaerobic–aerobic process are presented in Fig. 3. The influent NHþ 4 -N concentration rose from 472 to 1037 mg/l in the UASB reactor in the first five weeks, due to the degradation of organic nitrogenous compounds in the landfill reactor. The NHþ 4 -N removal efficiency dropped from 66% to 4% as the NHþ 4 N loading rate increased from 0.015 to 0.265 g/l d in the UASB reactor. Only small amounts of NHþ 4 -N were removed under the anaerobic condition owing to the utilization of NHþ 4 -N through assimilation of anaerobic bacteria for cellular growth (Kettunen et al., 1996). At the end of the experiment, no significant NHþ 4 -N removal was observed in the UASB reactor. On the contrary, the effluent NHþ 4 -N concentration sometimes exceeded the corresponding influent NHþ 4 -N concentration, due to the ammonification of organic nitrogenous compounds under anaerobic condition. The NHþ 4 -N removal efficiency was close to 100% in the ALSB reactor, but it dropped 12–29% when the OLR loading rate was higher than 0.77 g/l d between days 28 and 35. A sharp decrease in the effluent NO 3 -N concentration (day 35) also indicated the adverse effect of high OLR on nitrification. Therefore, it was important to ascertain the operating parameters of the sequential anaerobic–aerobic process for nitrification stability, because of the variation of leachate quality and volume during the course of landfill stabilization.
þ Fig. 3. NHþ NHþ 4 -N loading rates, 4 -N concentrations and NH4 -N removal efficiencies in the sequential anaerobic–aerobic process.
The effluent NO 3 -N concentration in the ALSB reactor was high at the beginning of the experiment, but it was at a low level during the rest period of operation. It implied that biological nitrogen removal took place in the ALSB reactor, because of the combination of aerated and non-aerated processes for the effluent with a high organic content from the UASB reactor (Priyali and Dentel, 1998). A high COD concentration might stimulate the faster growth of heterotrophic bacteria, which would consume excessive oxygen and nutrients (Patureau et al., 1997). As a result, a significant negative impact on the nitrification would be caused by the decreases of DO concentrations in both upflow and downflow zones, since the nitrifying bacteria are autotrophs, chemolithotrophs and obligate aerobes (Zhang et al., 2006) Simultaneous nitrification/denitrification has been previously found in the air-lift bioreactor for the treatment of wastewater (Guo et al., 2005). 3.3. Variation of nitrogenous compounds in leachate Leachate NO 3 -N concentrations showed a similar behavior in the experimental landfill reactors. Although the NO 3 -N concentration in recycled leachate reached 270 mg/l in the bioreactor landfill system, a low NO 3 -N concentration of less than 4 mg/l was obtained in leachate.
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This suggested that the landfill reactor had the capacity to deplete NO 3 -N. It was also observed that NO3 N concentrations were not detectable on day 6 after adding 500 and 1000 mg NO 3 -N to batch reactors filled with 1month-old refuse (Burton and Watson-Craik, 1998). Leachate NHþ 4 -N and TN concentrations in different landfill reactors are depicted in Fig. 4. There was no obvious difference in leachate NHþ 4 -N concentrations between the control and the bioreactor landfill system during the first 14 days. However, after this, the NHþ 4 -N concentration in leachate from the control increased dramatically to above 1000 mg/l and had high NHþ 4 -N concentration till the end of the experiment. As compared with the control, the leachate NHþ 4 -N concentration in the bioreactor landfill system increased slowly and reached the observed peak value of 1037 mg/l on day 35, then decreased quickly to below 200 mg/l on day 56. Leachate TN concentrations showed a similar range to NHþ 4 -N concentrations in both the two landfill reactors. After 56 days operation, the leachate TN concentration decreased to less than 200 mg/l in the experimental bioreactor landfill system. This indicated that an anaerobic–aerobic treatment in leachate recirculation had significant capability to in situ remove nitrogen in the bioreactor landfills. However, biological nitrogen removal processes were limited for the treatment of old leachate with low biodegradability (El-Fadel et al., 2002).
Fig. 5. Variation of leachate COD concentrations in different landfill reactors.
Leachate characteristics indirectly mirrored the biodegradation of organic refuse and the process of landfill stabilization (Tatsi and Zouboulis, 2002). In the experimental landfill reactors, leachate COD concentrations showed a similar trend, which increased in the first weeks, and then decreased gradually to below 1 g/l on day 105 (Fig. 5). The highest measured COD concentration occurred on day 21 and day 28, with values of 52.7 g/l in the bioreactor landfill system and 44.16 g/l in the control, respectively. At the late stage, the leachate COD concentration in the bioreactor landfill system decreased at a slower pace than in the control. The leachate COD concentration was less than
1 g/l until day 98 in the bioreactor landfill system, which was later in 14 days compared to the control. It was possibly explained that the high nitrate in recycled leachate after treated by the sequential anaerobic–aerobic reactors might inhibit the microbial activity involved in refuse decomposition (Price et al., 2003 ). Leachate VFA concentrations in the two landfill reactors presented a similar range to COD concentrations (Fig. 6). A faster decrease in leachate VFA concentration was achieved in the control relative to the bioreactor landfill system. Leachate pH values were in the range of 4.8–6.4 in the first 30 days of degradation in the experiment landfill reactors (Fig. 7). A peak leahcate pH value of 6.4 was observed on day 13 in the bioreactor landfill system and day 14 in the control, respectively. This might be attributed to the oxygen entrained in waste at burial, which had allowed the growth of aerobic bacteria during the early stage of waste decomposition. With the depletion of oxygen present initially (anaerobic condition prevailed in refuse ecosystem), soluble VFA intermediates such as acetate, butyrate and propionate appeared and increased in leachate. After day 35, leachate pH value increased with VFA hydrolyzed and fermented in subsequent steps to carbon dioxide and methane. Leachate pH value in the control reached above 6.5 on day 46, which was 26 days earlier than in the bioreactor landfill system. A similar result was observed by Price et al. (2003), in which a pH decrease and COD increase in
Fig. 4. Variation of leachate NHþ 4 -N and TN concentrations in different landfill reactors.
Fig. 6. Variation of leachate VFA concentrations in different landfill reactors.
3.4. Leachate characteristics
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lation will also depend on the favorable OLR for leachate treatment in the anaerobic reactor. Acknowledgements This work was financially supported by National Natural Science Foundation of China with Grant No. 50478083, and Ministry of Education Key Lab of Environment Remediation and Ecological Health with Grant No. 050001. Fig. 7. Variation of leachate pH value in different landfill reactors.
leachate were presented after their final nitrate addition. Similar observations were made by El-Mahrouki and Watson-Craik (2004) on the inhibitory effect of the recirculation of nitrified leachate through landfill sites on methanogenesis. The effects were dose-dependent, such that recovery of methane production was recorded within 5 and 23 days with added 100 and 750 mg/l NO 3 -N, respectively. Even after 24 days, no recovery was observed in cultures challenged with 1000 mg/l NO 3 -N. According to Roy and Conrad (1999) and Johannes et al. (2002) reports, the intermediates during denitrification such as NO and N2O may be the main reason for an inhibition of methanogenesis. This would likely result in the accumulation of carboxylic acids during the acid formation phase, and a slow decrease in the leachate VFA and COD concentrations.
4. Conclusion Nitrogen removal was feasible in the bioreactor landfill system with a sequential anaerobic–aerobic process treatment in leachate recirculation. After 56 days operation, the leachate TN and NHþ 4 -N concentrations decreased to less than 200 mg/l in the experimental bioreactor landfill system. In addition, the sequential anaerobic–aerobic process could effectively remove COD in leachate. When OLR was increased to 6.8–7.7 g/l d because of the degradation of landfilled refuse, a slight drop in the COD removal efficiency was found in the UASB reactor, but the total COD removal efficiency was not impaired by the increase in influent OLR and remained above 90%. However, a high COD concentration was observed in the effluent discharged from the UASB reactor. As a result, the nitrification was inhibited in the ALSB, since the nitrifying bacteria were autotrophs, chemolithotrophs and obligate aerobes. Therefore, in order to avoid nitrification inhibition in the aerobic ALSB reactor and reduce the costs derived from the anerobic COD removal, it would be necessary to have the internal leachate recirculation in landfills due to the variation of leachate quality and volume during landfill stabilization. The optimal volume necessary for nitrification and recircu-
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Characteristics of the bioreactor landfill system using an anaerobic–aerobic process for nitrogen removal Ruo He a
a,b,*
, Xin-wen Liu
b,c
, Zhi-jian Zhang b, Dong-sheng Shen
b
Ministry of Education Key Lab of Environment Remediation and Ecological Health, Zhejiang University, Hangzhou 310029, China b College of Environment and Resource, Zhejiang University, Hangzhou 310029, China c Department of Chemical engineering, Ningbo University of Technology, 20 Cuibai Road, NingBo 315016, PR China Received 18 April 2006; received in revised form 5 September 2006; accepted 10 September 2006 Available online 27 October 2006
Abstract A sequential upflow anaerobic sludge blanket (UASB) and air-lift loop sludge blanket (ALSB) treatment was introduced into leachate recirculation to remove organic matter and ammonia from leachate in a lab-scale bioreactor landfill. The results showed that the sequential anaerobic–aerobic process might remove above 90% of COD and near to 100% of NHþ 4 -N from leachate under the optimum organic loading rate (OLR). The total COD removal efficiency was over 98% as the OLR increased to 6.8–7.7 g/l d, but the effluent COD concentration increased to 2.9–4.8 g/l in the UASB reactor, which inhibited the activity of nitrifying bacteria in the subsequent ALSB reactor. The NO 3 -N concentration in recycled leachate reached 270 mg/l after treatment by the sequential anaerobic–aerobic process, but the landfill reactor could efficiently denitrify the nitrate. After 56 days operation, the leachate TN and NHþ 4 -N concentrations decreased to less than 200 mg/l in the bioreactor landfill system. The COD concentration was about 200 mg/l with less than 8 mg/l BOD in recycled leachate at the late stage. In addition, it was found that nitrate in recycled leachate had a negative effect on waste decomposition. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Anaerobic–aerobic process; Municipal solid waste; Bioreactor landfill; Biological nitrogen removal
1. Introduction Leachate recirculation is a new innovative practice in municipal solid waste (MSW) management because it reduces leachate organic strength, accelerates landfill stabilization, reduces landfill active life and increases landfill gas production (Reinhart and Townsend, 1998). However, recent findings have indicated that this practice negatively impacts the landfill environment. The increased level of biodegradation associated with leachate recirculation could result in the imbalance of the growth rates between fastgrowing acidogenic bacteria and slow-growing methano-
* Corresponding author. Address: Ministry of Education Key Lab of Environment Remediation and Ecological Health, Zhejiang University, Hangzhou 310029, China. Tel./fax: +86 571 86971156. E-mail address: [email protected] (R. He).
0960-8524/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2006.09.013
gens in the first phases of waste decomposition. As a result, methanogenesis may be delayed, prevented, or inhibited (O’Keefe and Chynoweth, 2000). In addition, with the waste decomposition and leachate recirculation, ammonia tends to accumulate and reach high concentrations in the anaerobic bioreactor landfill, which would inhibit anaerobic biodegradation of organic waste (Tada et al., 2005; Calli et al., 2006). The C/N ratio is lower in the stabilized leachate under the most conditions, and this has caused serious challenges to treatment systems (Trabelsi et al., 2000). Leachate ammonia is ordinarily removed by the way of ex situ treatment for bioreactor landfills. The methods for ammonia removal from stabilized leachate include ion exchange, air-stripping, chemical precipitation, reverse osmosis and biological nitrogen removal. Among these, biological treatment is the most common method for ammonia removal in leachate, due to the lower cost (Jokela
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et al., 2002; Uygur and Kargi, 2004). Conventional systems of biological nitrogen removal involve both nitrification and denitrification. For the high ammonia/low carbon wastewaster, organic matter limitation for denitrification appears, which is usually overcome by addition of external carbon sources such as methanol (Shiskowski and Mavinic, 1998). On the other hand, the process of biological nitrogen removal for leachate usually needs air-stripping pretreatment to adjust C/N ratio (Uygur and Kargi, 2004). This not only increases operational costs, but also enhances the difficulty of management. Recently, laboratory studies have showed the efficacy of in situ nitrogen removal in solid waste environment. Zhao et al. (2002) reported that 99.5% of the leachate ammonia was removed in a biofilter consisting of old waste (8–10 years old) with both anaerobic and aerobic sections. Onay and Pohland (1998) developed a three-component simulated landfill system, including anoxic, anaerobic and aerobic zones, to demonstrate the feasibility of in situ nitrification and denitrification at controlled landfills operated with leachate recirculation. The results demonstrated that both separate and combined reactor operations with international leachate recycling around each reactor provided 95% nitrogen conversion. In contrast, combined reactor operation, without internal recycling had conversion efficiency per cycle ranging from 30% to 52% for nitrification and from 16% to 25% for denitrification. Experiments by Berge et al. (2006) indicated that nitrification and denitrification may occur simultaneously in one aerobic landfill cell (even under low biodegradable C/N conditions). A landfill as a bioreactor for the conversion of nitrate to a harmless byproduct, nitrogen gas, is technically viable, and the denitrification reaction did not adversely affect the leachate pH (Price et al., 2003). The effectiveness of a combined ex situ nitrification and in situ denitrification process for managing nitrogen in leachate has been investigated (He et al., 2006). However, to date, landfill bioreactors for nitrogen removal from leachate with the variation of the quality and volume during landfill stabilization has been poorly understood. In the present study, the chemical oxygen demand (COD) and NHþ 4 -N removals were investigated in the anaerobic–aerobic process during landfill stabilization. In addition, the effects of an anaerobic–aerobic treatment in recycled leachate on nitrogen removal from leachate and waste stabilization were evaluated, compared to the bioreactor landfill system only with an anaerobic reactor using leachate recirculation. 2. Methods 2.1. MSW composition MSW was mixed by 10 different components shredded into 2–4 cm pieces in the experiment. The physical composition of the synthetic MSW mixture, according to the investigation made in the city of Ningbo, was as follows
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(by weight): vegetables, 45.0%; fish, 2.5%; meat, 1.0%; fruit, 9.0%; paper, 7.4%; plastics and leather rubber, 11.9%; cellulose textile, 3.6%; brick sand and soil, 8.5%; metals and glasses, 7.6%; woods, 3.5%. The moisture content of the mixture was about 54% (w/w). 2.2. Experimental set-up Three types of reactors used in this study are illustrated in Fig. 1. The simulated landfill reactor consisted of a 42-l cylinder made of PVC (28.7-cm i.d., 65-cm height). A polyethylene male adapter (about 0.8 cm) was installed at the bottom of each landfill reactor as a leachate drainage port. Two such adapters were installed in the lid of each landfill reactor for leachate recirculation and gas collection. Adapters were held in place with wax to provide a gas-tight system. The anaerobic reactor was operated in an upflow anaerobic sludge blanket (UASB) made of PVC (10-cm i.d., 80-cm height) with a working volume of 5.50 l. The aeration reactor was operated in an air-lift loop sludge blanket (ALSB), of which the working volume was 5.24 l. Several collection tanks with individual volume of about 3 l were attached to landfill, UASB and ALSB reactors for collections of leachate and effluent. 2.3. Experimental design and operation Two landfill reactors were used in the experiment. One was connected sequentially with a UASB reactor and an ALSB reactor by leachate recirculation in the experimental bioreactor landfill system. Fig. 1 shows its operational configuration. The other, only combined with a UASB reactor, was used as the control. The UASB reactors were seeded with raw anaerobic sludge procured from the Hangzhou citric acid factory and Hangzhou Shibao sewage treatment plant and 4.5 l of sludge was added to each of UASB reactors. The sludge was incubated with the synthetic water having a COD of 3000–5000 mg/l for 10 days to activate the sludge activity. Then it was acclimated by leachate from Hangzhou Tianzhiling landfill. The acclimated sludge in the UASB reactors of the bioreactor landfill system and the control contained total solids (TS) content of 107 g/l and 96.5 g/l and volatile solids (VS) content of 37.0 g/l and 34.5 g/l, respectively. The ALSB reactor was seeded with raw aerobic sludge procured from Hangzhou Shibao sewage treatment plant and 6 l of sludge, containing TS content of 14.0 g/l and VS content of 8.0 g/l, was added to the reactor. An aeration rate of 0.002 l/min m3 was introduced at the bottom of ALSB reactor. The dissolved oxygen (DO) was kept at 5 mg/l or so in the upflow zone, and about 3.8 mg/l in the downflow zone during the entire experiment. The sludge was acclimated by leachate with COD concentration of 200 mg/l and NHþ 4 -N concentration adjusted from 178 to 300 mg/l by NH4Cl. After ammonia removal efficiency reached 75% under the condition of the influent ammonia
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Wet gas meter
Pump
Sand
Collection tank Aeration
MSW Thermometer
Landfill reactor
UASB reactor
ALSB reactor
Fig. 1. Schematic diagram of the bioreactor landfill system in the experiment.
concentration and loading rate of 300 mg/l and 0.14 g/l d, the start-up of ALSB reactor was completed. Prior to filling, a 5 cm thickness of gravel was placed at the bottom of each landfill reactor to retain refuse and stop small particles from leaching out. Then about 23.4 kg synthetic MSW mixture, which was added with deionized water to 75% moisture content, was filled into each landfill reactor and a specific height of 50 cm was attained. Finally, the waste mixture was covered with a 5 cm depth of sand. Leachate was first treated in the UASB reactor, and the effluent subsequently was fed to the ALSB reactor. The effluent discharged from the ALSB reactor was recirculated into the landfill reactor for denitrification. Leachate was continuously circulated among the landfill, UASB and ALSB reactors by using pumps with adjusted flow rates changing with leachate volume during waste decomposition, except for the first two days when no recycled lechate was fed to the landfill reactor. The simulated landfill reactors were operated at room temperature and the UASB and ALSB reactors were carried out in a temperature-controlled room at 30 ± 1 °C. 2.4. Analytical methods Samples were collected weekly from the effluent port of each reactor. COD, total nitrogen (TN), NHþ 4 -N, NO3 -N and pH were determined by conventional methods (EPA of China, 1989). Volatile fatty acids (VFA) were analyzed by acidified ethylene glycol colorimetric method (CBICAS, 1984).
Fig. 2. OLRs, COD concentrations and COD removal efficiencies in the sequential anaerobic–aerobic process.
3. Results and discussion 3.1. COD removal in the sequential anaerobic–aerobic process Fig. 2 shows the temporal variations of organic loading rates (OLRs), COD concentrations and COD removal efficiencies in the sequential anaerobic–aerobic process. With the degradation of landfilled refuse, the influent COD concentration in the UASB reactor increased at the early stage, and reached the maximum value of 52.7 g/l on day 21, then
decreased to 560 mg/l on day 105. The curve of OLRs changing over time in the UASB reactor was similar to that of COD concentration. However, the highest OLR occurred on day 28, which was later in a week compared with the highest influent COD concentration. This could be attributed to more leachate produced on day 28 than that on day 21, due to waste decomposition. When the OLR was increased to 6.8–7.7 g/l d between days 28 and 35, the effluent COD concentration reached 2.9–4.8 g/l, although the COD removal efficiency was still above 88%
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in the UASB reactor. Previous studies have also shown that COD concentration in effluent discharged from anaerobic reactor was increased while OLRs were increased step by step (Im et al., 2001). The COD removal efficiency was increased to 98% as the OLR increased from 0.04 to 0.95 g/l d in the ALSB reactor. The total COD removal efficiency in the whole system was not affected by the increase of influent OLR, and remained above 90% during the first 63 days. When the influent COD concentration decreased to below 1000 mg/l in the UASB reactor, the COD removal efficiency dropped to about 14–40%. This was consistent with previous report in anaerobic treatment of leachate generated from old and young landfills (Me´ndez et al., 1989). Since the residual COD in the effluent discharged from the UASB reactor was continuously metabolized in the ALSB reactor, the total COD removal efficiency was maintained above 65% in the anaerobic–aerobic process. The effluent COD concentration remained 200 mg/l or so in the ALSB reactor at the late stage, due to a low biodegradability of leachate organics with less than 8 mg/l biochemical oxygen demand (BOD) (data not shown). This was similar with the effluent inert COD concentration in the sequential two-stage UASB/ completely stirred tank reactor (CSTR) system for the treatment of landfill leachate (Ag˘dag˘ and Sponza, 2005). 3.2. NHþ 4 -N removal in the sequential anaerobic–aerobic process NHþ 4 -N
NHþ 4 -N
The variations of loading rates, concentrations and NHþ -N removal efficiencies in the sequen4 tial anaerobic–aerobic process are presented in Fig. 3. The influent NHþ 4 -N concentration rose from 472 to 1037 mg/l in the UASB reactor in the first five weeks, due to the degradation of organic nitrogenous compounds in the landfill reactor. The NHþ 4 -N removal efficiency dropped from 66% to 4% as the NHþ 4 N loading rate increased from 0.015 to 0.265 g/l d in the UASB reactor. Only small amounts of NHþ 4 -N were removed under the anaerobic condition owing to the utilization of NHþ 4 -N through assimilation of anaerobic bacteria for cellular growth (Kettunen et al., 1996). At the end of the experiment, no significant NHþ 4 -N removal was observed in the UASB reactor. On the contrary, the effluent NHþ 4 -N concentration sometimes exceeded the corresponding influent NHþ 4 -N concentration, due to the ammonification of organic nitrogenous compounds under anaerobic condition. The NHþ 4 -N removal efficiency was close to 100% in the ALSB reactor, but it dropped 12–29% when the OLR loading rate was higher than 0.77 g/l d between days 28 and 35. A sharp decrease in the effluent NO 3 -N concentration (day 35) also indicated the adverse effect of high OLR on nitrification. Therefore, it was important to ascertain the operating parameters of the sequential anaerobic–aerobic process for nitrification stability, because of the variation of leachate quality and volume during the course of landfill stabilization.
þ Fig. 3. NHþ NHþ 4 -N loading rates, 4 -N concentrations and NH4 -N removal efficiencies in the sequential anaerobic–aerobic process.
The effluent NO 3 -N concentration in the ALSB reactor was high at the beginning of the experiment, but it was at a low level during the rest period of operation. It implied that biological nitrogen removal took place in the ALSB reactor, because of the combination of aerated and non-aerated processes for the effluent with a high organic content from the UASB reactor (Priyali and Dentel, 1998). A high COD concentration might stimulate the faster growth of heterotrophic bacteria, which would consume excessive oxygen and nutrients (Patureau et al., 1997). As a result, a significant negative impact on the nitrification would be caused by the decreases of DO concentrations in both upflow and downflow zones, since the nitrifying bacteria are autotrophs, chemolithotrophs and obligate aerobes (Zhang et al., 2006) Simultaneous nitrification/denitrification has been previously found in the air-lift bioreactor for the treatment of wastewater (Guo et al., 2005). 3.3. Variation of nitrogenous compounds in leachate Leachate NO 3 -N concentrations showed a similar behavior in the experimental landfill reactors. Although the NO 3 -N concentration in recycled leachate reached 270 mg/l in the bioreactor landfill system, a low NO 3 -N concentration of less than 4 mg/l was obtained in leachate.
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This suggested that the landfill reactor had the capacity to deplete NO 3 -N. It was also observed that NO3 N concentrations were not detectable on day 6 after adding 500 and 1000 mg NO 3 -N to batch reactors filled with 1month-old refuse (Burton and Watson-Craik, 1998). Leachate NHþ 4 -N and TN concentrations in different landfill reactors are depicted in Fig. 4. There was no obvious difference in leachate NHþ 4 -N concentrations between the control and the bioreactor landfill system during the first 14 days. However, after this, the NHþ 4 -N concentration in leachate from the control increased dramatically to above 1000 mg/l and had high NHþ 4 -N concentration till the end of the experiment. As compared with the control, the leachate NHþ 4 -N concentration in the bioreactor landfill system increased slowly and reached the observed peak value of 1037 mg/l on day 35, then decreased quickly to below 200 mg/l on day 56. Leachate TN concentrations showed a similar range to NHþ 4 -N concentrations in both the two landfill reactors. After 56 days operation, the leachate TN concentration decreased to less than 200 mg/l in the experimental bioreactor landfill system. This indicated that an anaerobic–aerobic treatment in leachate recirculation had significant capability to in situ remove nitrogen in the bioreactor landfills. However, biological nitrogen removal processes were limited for the treatment of old leachate with low biodegradability (El-Fadel et al., 2002).
Fig. 5. Variation of leachate COD concentrations in different landfill reactors.
Leachate characteristics indirectly mirrored the biodegradation of organic refuse and the process of landfill stabilization (Tatsi and Zouboulis, 2002). In the experimental landfill reactors, leachate COD concentrations showed a similar trend, which increased in the first weeks, and then decreased gradually to below 1 g/l on day 105 (Fig. 5). The highest measured COD concentration occurred on day 21 and day 28, with values of 52.7 g/l in the bioreactor landfill system and 44.16 g/l in the control, respectively. At the late stage, the leachate COD concentration in the bioreactor landfill system decreased at a slower pace than in the control. The leachate COD concentration was less than
1 g/l until day 98 in the bioreactor landfill system, which was later in 14 days compared to the control. It was possibly explained that the high nitrate in recycled leachate after treated by the sequential anaerobic–aerobic reactors might inhibit the microbial activity involved in refuse decomposition (Price et al., 2003 ). Leachate VFA concentrations in the two landfill reactors presented a similar range to COD concentrations (Fig. 6). A faster decrease in leachate VFA concentration was achieved in the control relative to the bioreactor landfill system. Leachate pH values were in the range of 4.8–6.4 in the first 30 days of degradation in the experiment landfill reactors (Fig. 7). A peak leahcate pH value of 6.4 was observed on day 13 in the bioreactor landfill system and day 14 in the control, respectively. This might be attributed to the oxygen entrained in waste at burial, which had allowed the growth of aerobic bacteria during the early stage of waste decomposition. With the depletion of oxygen present initially (anaerobic condition prevailed in refuse ecosystem), soluble VFA intermediates such as acetate, butyrate and propionate appeared and increased in leachate. After day 35, leachate pH value increased with VFA hydrolyzed and fermented in subsequent steps to carbon dioxide and methane. Leachate pH value in the control reached above 6.5 on day 46, which was 26 days earlier than in the bioreactor landfill system. A similar result was observed by Price et al. (2003), in which a pH decrease and COD increase in
Fig. 4. Variation of leachate NHþ 4 -N and TN concentrations in different landfill reactors.
Fig. 6. Variation of leachate VFA concentrations in different landfill reactors.
3.4. Leachate characteristics
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lation will also depend on the favorable OLR for leachate treatment in the anaerobic reactor. Acknowledgements This work was financially supported by National Natural Science Foundation of China with Grant No. 50478083, and Ministry of Education Key Lab of Environment Remediation and Ecological Health with Grant No. 050001. Fig. 7. Variation of leachate pH value in different landfill reactors.
leachate were presented after their final nitrate addition. Similar observations were made by El-Mahrouki and Watson-Craik (2004) on the inhibitory effect of the recirculation of nitrified leachate through landfill sites on methanogenesis. The effects were dose-dependent, such that recovery of methane production was recorded within 5 and 23 days with added 100 and 750 mg/l NO 3 -N, respectively. Even after 24 days, no recovery was observed in cultures challenged with 1000 mg/l NO 3 -N. According to Roy and Conrad (1999) and Johannes et al. (2002) reports, the intermediates during denitrification such as NO and N2O may be the main reason for an inhibition of methanogenesis. This would likely result in the accumulation of carboxylic acids during the acid formation phase, and a slow decrease in the leachate VFA and COD concentrations.
4. Conclusion Nitrogen removal was feasible in the bioreactor landfill system with a sequential anaerobic–aerobic process treatment in leachate recirculation. After 56 days operation, the leachate TN and NHþ 4 -N concentrations decreased to less than 200 mg/l in the experimental bioreactor landfill system. In addition, the sequential anaerobic–aerobic process could effectively remove COD in leachate. When OLR was increased to 6.8–7.7 g/l d because of the degradation of landfilled refuse, a slight drop in the COD removal efficiency was found in the UASB reactor, but the total COD removal efficiency was not impaired by the increase in influent OLR and remained above 90%. However, a high COD concentration was observed in the effluent discharged from the UASB reactor. As a result, the nitrification was inhibited in the ALSB, since the nitrifying bacteria were autotrophs, chemolithotrophs and obligate aerobes. Therefore, in order to avoid nitrification inhibition in the aerobic ALSB reactor and reduce the costs derived from the anerobic COD removal, it would be necessary to have the internal leachate recirculation in landfills due to the variation of leachate quality and volume during landfill stabilization. The optimal volume necessary for nitrification and recircu-
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